Glass cloth laminates, useful as core layers for printed circuit boards, utilizing epoxy resin impregnating compositions are well known, and taught, for example, by Alvino et al., in U.S. Pat. No. 4,327,143. In the Alvino et al. compositions, flame retardants, such as chlorinated phenols, could be included, and the epoxy resin was cured primarily by a monomethylol dicyandiamide-methyl Cellosolve solution. Co-curing agents could include 0 wt. % to 2 wt. %, based on monomethylol dicyandiamide, of an amine, such as benzyl dimethylamine, pyridine, quinoline, pyrazine, and single carbon mono substituted imidazoles, such as imidazole, 1 methyl imidazole, and 2 methyl imidazole. Potassium carbonate or tetramethyl ammonium hydroxide could be used as accelerators.
The resin system used in Alvino et al., while eliminating moisture crystallization problems in the core, could be improved in terms of gel time, and thermal stability, i.e., glass transition temperatures, Tg (DSC) value, which is about 114.degree. C. to 125.degree. C. for molded, cured laminates. Also, its flow characteristics are not suitable for coating very thin glass cloth substrates used as adhesive spacers in multilayer circuit boards. Additionally, use of a benzyl dimethylamine type co-curing agent requires substantial aging of the resin system before use, which requires special storage facilities, and which is expensive and inconvenient in commercial operation.
There has been a long felt need for less expensive resin systems that can be used in manufacturing high temperature multilayer circuit boards, where the resin better wets and coats glass cloth substrates, especially thin spacer substrates, where thermal stability, i.e., Tg (DSC) values, of molded, cured impregnated laminates are above about 150.degree. C., where impregnated, "B"-staged pre-preg substrates maintain several months storage stability and good resistance to crystallization, and where faster pre-preg processing times result.